Mine upgraded to Android 2.2 OTA (Over The Air) about a month ago, and it started having reboot issues - it would suddently turn off and get into a reboot cycle where it would start, display the splash screen, reboot, start, display the splash screen, reboot, etc etc.
Only happened once in a while though, and infrequently enough to put up with it.
However, last week another OTA upgrade came through and that's caused it to reboot constantly - it never starts up, it just keeps rebooting - so beware when you do the upgrades guys! It's been with HTC for the past week to be "serviced" thopugh I have a feeling they'll just flash it back down to the original software version...